1. Field of the Invention
The present invention is related to a vital signal detecting apparatus capable of detecting a vital signal containing no outer disturbance from pulse waves, and also related to a noninvasive blood pressure meter adopting a system for measuring a variation in blood pressure.
2. Description of the Related Art
Conventionally, such blood pressure meters capable of continuously measuring blood pressure with employment of cuffs are known in the medical electronic field. In this type of conventional blood pressure meter, when a measuring time interval is shortened, fastening actions by the cuff are frequently repeated. As a result, heavy loads could be given to a human subject under examination, and internal hemorrhages may happen to occur in a human body portion around which the cuff is wound in the worst case.
In contrast to this type of blood pressure meters, there is another type of blood pressure meter capable of measuring only a variation in blood pressure, which does not directly measure an absolute value of blood pressure. For example, while this type of blood pressure meter grasps speeds at which pulse waves are propagated through an artery (pulse wave propagation speeds) and also grasps feature amounts of pulse waveforms to thereby constitute parameters thereof, a change amount thereof is converted into a change amount in blood pressure and then the blood pressure meter displays this converted blood pressure amount. Since this sort of blood pressure meter needs not repeat fastening actions by the cuff, no heavy load is given to the human subject under examination. Also, since this blood pressure meter can calculate in principle the blood pressure in response to one cardiac beat of ECG and pulse wave signals, blood pressure measurement can be carried out continuously under noninvasive condition.
However, the above-explained blood pressure meter, namely the blood pressure meter capable of measuring only the blood pressure variation, calculates the blood pressure based upon the change amount in the pulse waves. Therefore, if the outer disturbance is superimposed on the pulse waveforms due to the body movement (motion of living body), then this blood pressure meter owns such a problem that blood pressure measurement cannot be carried out correctly.
To solve this problem, the below-mentioned conventional techniques have been proposed.
(1) Japanese Patent No. 2664983 discloses a vital signal detecting apparatus having judgment means for determining as to whether or not body movement noise is produced. In the case that this judgment means judges the production of such body movement noise, lowering of the blood pressure measuring precision, which is caused by reading the body movement noise, can be prevented by such a manner that reading operation of sensor signals is interrupted for a predetermined time duration.
(2) Japanese Laid-open Patent Application No. 5-184547 describes a method for calculating a basic period of a present pulse wave by utilizing periods of plural pulse waves, and for correcting the amplitude of an abnormal pulse wave in response to fluctuation of the pulse wave period that is calculated from this basic period.
(3) A document of xe2x80x9cComparison on relationship between pulse wave propagation time and blood pressure of different measuring portionsxe2x80x9d (Japanese ME institute No. 38, Sendai, 21 to 23 April, 1999, page 373) describes a method capable of removing outer disturbance caused by the body movement from the pulse waves of the human body. In other words, this method is capable of removing adverse influences of general artifacts containing the body movement, from the pulse waves and the electrocardiogram. Concretely, while observing the 64 cardiac beats of the pulse waves appeared in the past from the monitoring time of the pulse waves, a portion having large dispersion of the pulse wave amplitude is removed, and the signal portions other than this removed portion are used for the analysis purpose.
However, the conventional method of the above-explained item (1) is not properly applied to such a blood pressure measurement under free action, in which the body movement noise is frequently superimposed on the vital signal.
The conventional method of the above-explained item (2) may be utilized as to a single abnormal pulse wave. However, this method can hardly calculate the basic frequency of the pulse wave under free action in which the pulse wave is frequently disturbed. Similarly to the conventional method of the item (1), this method of the item (2) is not suitable for the blood pressure measurement under free action.
In accordance with the conventional method of the above-explained item (3), since the signal portion on which the outer disturbance caused by the body movement is superimposed is not used, there is a limitation in the vital signal that can be used in the analysis. Also, the temporal continuity of the vital signal would be lost. In addition, when the outer disturbance caused by the body movement is applied for a long time duration, the resulting vital signal cannot be analyzed.
As a result, as to the noninvasive blood pressure meter adopting the system for measuring only the blood pressure variation, in such a case that the pulse waveforms are largely disturbed due to the body movement that is frequently produced under free action, the sufficiently high accuracy of predicting the blood pressure cannot be secured.
The present invention has been made in view of the above problems. An object of the present invention is to provide a vital signal detecting apparatus capable of detecting a vital signal, containing no outer disturbance, from pulse waves, and also capable of maintaining temporal continuity of the vital signal. Another object of the present invention is to provide a noninvasive blood pressure meter adopting a system of measuring only a blood pressure variation, and capable of measuring blood pressure continuously and precisely even under free action by separating noise, caused by body movement, from pulse wave components synchronized with cardiac beats.
According to the present invention, a vital signal detecting apparatus has at least two sensors for detecting first pulse waves and outputting analog signals corresponding to the first pulse waves; an A/D converter for converting the analog signals derived from the sensors into digital signals; a separation matrix calculation means for calculating a separation matrix for separating an outer disturbance caused by body movement, from the first pulse waves by utilizing statistic independency of a second pulse wave containing no outer disturbance and the outer disturbance; and a signal separation calculation means for separately deriving a vital signal corresponding to the second pulse wave and a body movement signal indicating the outer disturbance by operating the separation matrix with respect to the digital signals. Accordingly, the vital signal containing no outer disturbance can be detected from pulse waves continuously.
Specifically, the separation matrix calculation means can calculate the separation matrix in a case where it is considered that each of the first pulse waves detected by the sensors contains the outer disturbance and the second pulse wave containing no outer disturbance, and only amplitude is changed until the sensors detect the first pulse waves. Otherwise, the separation matrix calculation means can calculate the separation matrix in a case where it is considered that each of the first pulse waves detected by the sensors contains the outer disturbance caused by body movement and the second pulse wave containing no outer disturbance, and both frequency and phase thereof are changed linearly until the sensors detect the first pulse waves.